We propose to develop a broadly neutralizing, possibly universal influenza vaccine based on virus-like particles (VLPs) displaying remodeled HA molecules which present otherwise cryptic epitopes. These remodeled HAs will be expressed in forms lacking the dominant hypervariable epitopes and instead display distinctly conserved subdominant antigenic sites known to elicit an antibody response that will neutralize a broad spectrum of influenza viruses. The protective scope of current influenza vaccines is restricted to homologous viruses or closely related variants and vaccine efficacy wanes following the fast antigenic evolution of the influenza virus. Most protective antibodies target highly variable and dominant sites on the globular head of the HA molecule, although more conserved and less immune-recognized conformational antigenic sites are also present in the stem (HA2) and between the globular head (HA1) and stem portions of HA. Isolated human antibodies directed toward these sites have been found to neutralize a broad spectrum of influenza viruses. It seems reasonable therefore to prepare and test vaccines that display these highly conserved subdominant antigenic sites and determine if they stimulate a broad antibody response which is minimal in a natural influenza infection or following vaccination with formulations containing whole HA molecules. Incorporation of remodeled HA molecules into influenza virus-like particles (VLPs) should provide an excellent opportunity to develop a broadly neutralizing vaccine. VLPs are generated by the co-expression of four structural influenza proteins (M1, M2, HA and NA) and do not contain viral genetic material and are therefore unable to replicate or cause infection. VLPs displaying different remodeled HA molecules will be produced, characterized and tested for the presence of conserved epitopes by immunoprecipitation with specific antibodies recognizing these sites. The neutralizing activity of VLP immunized mice sera will be assessed by an in-vitro micro- neutralization assay using three antigenically distinct viruses. The protective efficacy and immunogenicity stimulated by a single or combined candidate vaccine will be further investigated in lethal challenge studies with three antigenically diverse viruses. Further development of the most promising VLP(s) will be pursued by a phase II SBIR proposal.